Industrially important chemical processes include ammonia synthesis from nitrogen and hydrogen, hydrogen peroxide synthesis from oxygen and hydrogen, and hydrocarbon synthesis from carbon monoxide or carbon dioxide and hydrogen. All of these reactions are energy intensive.
Ammonia is industrially synthesized from the elements hydrogen and nitrogen by the Haber process in which nitrogen and hydrogen are exothermically reacted over an iron catalyst at elevated pressures, e.g. from about 100 to about 1000 atmospheres and generally from about 200 to about 300 atmospheres, and at elevated temperatures, e.g. from about 400.degree. to about 550.degree. centigrade and generally from about 450.degree. to 500.degree. degrees centigrade. The iron catalyst contains reduced oxides of iron that are doubly promoted, that is promoted with an oxide such as alumina, silica, or zirconia, and an oxide of an alkali metal or alkaline earth metal as potassium oxide or calcium oxide.
The compression of nitrogen and hydrogen gases are energy intensive processes. Moreover, the high temperatures required for the reaction provide only limited opportunities to recapture the energy of compression in other processes in an industrial chemical process.
Conventional industrial processes for the production of hydrogen peroxide use either the cyclic oxidation and reduction of hydroquinone to produce anthraquinone and hydrogen peroxide, or the direct electrochemical reduction of oxygen to hydrogen peroxide at a cathode.
In coal gasification processes, carbon monoxide and carbon dioxide are the initial intermediates, obtained by the heating of coal in the presence of steam and air under carefully controlled conditions. High temperature and high pressure catalyzed reactions, e.g., the Fisher-Tropsch reaction, of carbon dioxide or carbon monoxide with hydrogen produce a variety of hydrocarbon products, e.g. alkanes, alkenes, and other products.
Various methods of direct and indirect electrochemical reactions have also been studied to convert carbon dioxide to hydrocarbon products while avoiding high temperatures and pressures of the conventional catalytic processes.
The direct electrochemical reduction of carbon dioxide has been studied by S. Kopusta, and N. Hackerman, in Journal of the Electrochemical Society, Volume 130, pages 607 to 613 (1983). As there described, carbon dioxide is reduced and then reacts with a proton donor to produce formate. The current efficiency is high, generally about 95%, but the exchange current density is extremely low, generally about 5.times.10.sup.-11 Amperes/cm2. This shows that the rate of reaction is low. Furthermore, the efficiency of reaction decreases as the total current through the cell increases. As the reaction of carbon dioxide must compete with the reduction of protons, electrode materials with high hydrogen overpotentials, e.g. mercury, tin, indium, or titanium dioxide must be used.
Indirect cathodic reduction of carbon dioxide has been studied by B. Fisher and R. Eisenberg, in the Journal of the American Chemical Society, Volume 102, Pages 63 to 7363 (1980) by I. S. Kolmitikov, et al, in Izu. Akad, Nauk S. S. R. Ser. Khim., Volume 1970, Page 26-50 and Volume 1972, Page 22-29, and by G. O. Evans, and C. J. Wewell in Inorganic Chim Acta., Volume 31, Pages L387-L390 (1978). The indirect reduction has been accomplished with cobalt and nickel tetraazamacrocycles, transition metal phosphine complexes, anion carbonyl hydrides and dinuclear carbonyls. The overvoltage for reduction of the complexes is less than that required for direct cathodic reduction of carbon dioxide. However, the stability of the complexes is not adequate for repeated oxidation and reduction cycles.
The reason that the indirect electrochemical reduction occurs at lower overvoltages may be that the carbon dioxide bonds are distorted by bonding to metal complexes. Carbon dioxide acts as a Lewis base with the lowest electron density being at the central carbon. Complexes with electron rich metal atoms thus bind the carbon atom.
Each of the above reactions for the synthesis of ammonia, hydrogen peroxide, and hydrocarbons involve hydrogenation of nitrogen, oxygen and carbon monoxide or carbon dioxide, respectively.
Electrochemical reduction and hydrogenation of these compounds at a cathode is inherently difficult because electrostatic repulsion between the reactant and the negatively charged cathode hinders adsorption of the reactant on the electrode and thereby limits the rate of reaction.